The present invention relates to fabrication of field emitters, particularly to sharpening of field emitter tips, and more particularly to sharpening the tips using high-energy ions incident along or near the longitudinal axis of the field emitter.
In field emission flat-panel displays, the emitted current for a certain applied gate voltage is strongly influenced by the sharpness of the emitter tip. This is a consequence of the electric field enhancement that results from decreasing tip radius or sharpness and the shape of the shank of the emitter below the tip. The effect is present independent of the nature of the tip structure (e.g., cones, filaments, pyramids, or wires). As the dimensions of the emitters and associated elements (e.g., gate diameters) decreases, sharpening of the emitter tips becomes more difficult and, usually, is not performed. However, increasing the sharpness of the tips reduces the gate voltage required for a given emission current and, consequently, the required power. The power required to operate the emission part of the flat panel display is the product of the capacitance and the square of the gate voltage; thus, there is significant incentive to decrease the gate voltage.
Various prior efforts have been directed to sharpening the field emitter tips. Previous attempts to sharpen such tips using ions were limited to low-energy ions (e.g., less than 10 keV) or to focused ion beams. These low-energy ion and focused ion approaches are respectively exemplified by O. Auciello et al., "Ion Bombardment Sharpening of Field Emitter Arrays", International Vacuum Microelectronics Conference, Abstracts, p. 192-196 (1995); and M. Takai et al., "Modification of Field Emitter Array (FEA) Tip Shape by Focused Ion Beam Irradiation", International Vacuum Microelectronics Conference, Abstracts, p. 52-55 (1995). Low-energy ions provide some sharpening, but the sharpness is limited by lower sputtering yields (i.e., number of atomic ejections per ion) and to the region near the top of the tip and does not modify the shape of the tip sufficiently to maximize the field enhancement factor. The use of focused ion beams is constrained by the diameter of the focused ion beam and is a serial sharpening process, which is not an acceptable manufacturing option because of the large number of emitters required (i.e., more than 500 emitters per pixel).
Recently, efforts have been directed to fabricating field emitters with sharp emitting points, wherein the metal emitters are formed by electroplating, and the shape of the formed emitter is controlled by the potential imposed on the gate as well as on a separate counter electrode. Such a process is described and claimed in copending U.S. application Ser. No. 08/847,086, filed May 1, 1997, entitled "Electrochemical Formation of Field Emitters", assigned to the same assignee. Also, sharpening of previously formed field emitter tips has been carried out, wherein tip sharpening is done by electroetching/polishing using the grid (gate) of the field emission structure, such as a field emission triode structure, as a counter electrode. Such a process is described and claimed in copending U.S. application Ser. No. 08/847,087 filed May 1, 1997 entitled "Electrochemical Sharpening of Field Emission Tips", assigned to the same assignee.
The present invention provides a more efficient process for sharpening field emitter tips and is adaptable to manufacturing operations for fabricating tips of gated arrays. This is accomplished by using high-energy (30 keV or more) ion beams rastered over the arrays using standard ion implantation equipment. The high-energy ion are incident along or near the longitudinal axis of the field emitters to sharpen the tips with a taper from a sharp point down to the shank of the emitter. The process will sharpen tips down to radii of less that 12 nm with an included angle of about 20 degrees. Also, the process can be utilized to shorten the length of the field emitter after the tip has been sharpened.